High print quality of xerographic reproductions is desired. The print quality depends strongly on the photoreceptor coating quality. For coating from a solution, the coating thickness uniformity depends on how the coating conditions have been optimized with the rheological and interfacial properties of the solution. Solutions containing solvent, monomers, polymers, and/or pigments are often used for coating. Many of the solutions change their rheological properties or microscopic structures (especially for pigment dispersions) under shear deformation. It is therefore important to design a coating method to accommodate such change under shear deformation to achieve the required coating uniformity. For dip coating, the coating vessel or tank design is important to obtain good coating quality. The detailed design of the geometry and size of the vessel and the accessories such as the atttached piping and the pump determines the solution flow pattern which can affect the final coating quality especially for non-Newtonian solutions. Newtonian flow is flow of a liquid having a constant viscosity under shear deformation. Viscosity of non-Newtonian liquid may decrease or increase with increasing shear deformation or shear rate, i.e., so-called shear thinning or shear-thickening.
Ideally, the use of Newtonian solutions for coating can avoid a lot of coating problems, but practically it is difficult to develop such solutions which must also satisfy other performance requirements such as xerographic properties. Most of the time, non-Newtonian solutions must be used for coating. However, such non-Newtonian solutions with all the required performance properties may not be ideal for coating. Many of them change in viscosity upon the application of shear and the dispersed particles in some solutions aggregrate and settle at low shear rates. Often the solutions have yield stress, below which the dispersion shows little or no deformation, i.e., little or no flow. Newtonian flow does not have a yield stress. Liquid with a yield stress is one type of a non-Newtonian liquid. Some non-Newtonian liquids do not have yield stress. At very low yield stress or low shear rate level, the viscosity reaches extremely high levels. It has been observed that a non-Newtonian dispersion with shear thinning, yield stress, and flocculating properties can cause coating non-uniformities such as streaking, marbling, and sloping (that is, a top to bottom thickness difference on a drum).
The present invention addresses the above stated problem with non-Newtonian solutions by providing a method to select the appropriate gap distance employed between the substrate and the dip coating vessel wherein the selected gap distance will minimize or eliminate coating non-uniformities.
Dip coating methods and apparatus are disclosed in Mistrater et al., U.S. Pat. No. 5,693,372; Mistrater et al., U.S. Pat. No. 5,681,391; Petropoulos et al., U.S. Pat. No. 5,578,410; and Petropoulos et al., U.S. Pat. No. 5,633,046, the disclosures of which are totally incorporated herein by reference.
In addition, application Ser. No. 09/182,087, having the inventor Jian Cai, discloses an extrusion coating process where there is maintained an applied shear stress to the coating composition that is greater than the yield stress of the coating composition.